Windows 10 Pro 64bit, AMD Ryzen 5 2600, 16Gb RAM
NVidia GTX 1060 6Gb
Windows 10 Pro 64bit, AMD Ryzen 5 1600, 8Gb RAM
NVidia GTX 1050Ti 4 Gb
Blender 2.8 Beta 91a155833e59
If you are texture painting in one window and you modify the shader by adding/removing an image texture to it in another, it won't appear/disappear from the texture slot list until you change to edit mode (or anything else) and back. Another issue is that this way you can paint onto an image texture that is no more connected to the shader and it doesn't disappear even if you do the "edit mode and back" (TAB x2) trick.
Steps: Add an image texture to the shader in the attached file, connect to the roughness for example, create a new texture for. It won't be in the texture slots list. Now TAB x2 in the 3D view, slot appears. Paint on it, delete the node from the shader. It is still in the list. TAB x2. It is no more in the list, but the 3D view still shows the texture.